Sample Preparation: Making Pressed Pellets · 2.) Making a Pellet Using a Cement Sample as an...

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Sample Preparation: Making Pressed Pellets

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1.) Introduction

Pressing a sample leads to a pressed pellet with a defined density thus ensuring reproducible

sample preparation.

For X-ray fluorescence analysis, it is important that the pellet is also mechanically stable

because when being introduced into the analytical instrument, the sample is subjected to

evacuation and then aeration after the measurement. High stability is required so that the

sample does not break during these procedures. A few materials display this stability which

can be increased by pressing in aluminum cups or rings. However, mixing with a binder is

obligatory for most materials.

Using a cement sample as an example, this Whitepaper presents the steps for creating the

perfect pressed pellet and provides important tips for sample preparation.

2.) Making a Pellet Using a Cement Sample as an Example

Example: Preparation instructions for a 32 mm pressed pellet:

Weigh

4.00 g Sample

1.00 g CEREOX®

into a mixing cup, add 1 mixing ball and mix for 2 minutes in the MUK mixer. Then place a

pellet film on the press plate, remove the mixing ball and press.

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2. 1) Selecting the Press

The 15 tons of the PR-15 are completely sufficient for making a pressed pellet with a 32 mm

diameter. As described above, it is most important that the production of pressed pellets is

reproducible. VANEOX presses ensure that each powder is pressed with exactly the same

pressure; giving the pellets a defined sample density.

Because pressure is equal to force (tons) per area, at least 20 tons, e.g., from the PR-25A, PR-

25N or PR-40Da, should be used to achieve comparable densities for larger diameters, such as

40 mm.

The PR-25A differs from the PR-25N in that the stroke of the cylinder is larger. This is required

when samples have a very large volume and a very small mass so that pressing can be

performed in one step. Otherwise, several steps are necessary.

A problem occurs when the pressing procedure is conducted too quickly, as trapped air causes

parts of the pellet to flake later. Too much force can also lead to the pressed pellet remaining

stuck to the pressing plate; this can only be helped by reducing the force or using pellet films.

Figure 1 Comparison of different presses



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The programable 40-ton press, PR-40, overcomes these problems by slowly building up the

pressure in up to three steps, whereby it is also possible to specify a waiting time after each

pressing step.

The PR-40’s integrated pressing die considerably simplifies cleaning, thus leading to higher

throughput. The throughput can be further increased by using rings because the last step of

pressing out of the pellet is eliminated.

However, for an increase in pressure it is necessary to take the load bearing capacity of the

pressing die into account:

2.2) Selecting the Pressing die

Load limits for pressing dies without rings

Diameter Maximum pressing force

ab 40 mm 40 t

35 mm – 39 mm 30 t

32 mm – 34 mm 25 t

28 mm – 31 mm 18 t

15 mm – 27 mm 5.0 t

10 mm – 14 mm 2.5 t

8 mm 1.5 t

Load limits for pressing dies with rings

With ring (outer/inner diameter in mm) Maximum pressing force

PR-CD-6032R (40/32) 15 t

PR-CD-6016R (20/16) 5.0 t

PR-40-CD35R_b (40/35) 15 t

PR-40-CD35R (51,5/35) 30 t

PR-40-CD32R (40/32) 15 t



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2.3) Grinding the Sample

A grain size of less than 100 µm is recommended for XRF analysis of pressed pellets. The

sample can be ground in different ways to achieve this grain size. The following lists show the

different principles:

Figure 2 Principle: Grinding a sample

Figure 3 Principle: Grinding a sample



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2.4) Weighing in the Sample

When the sample has been ground, the necessary amount is weighed.

Figure 4: Weighing the sample

2.5) Binding Material: Overview of the binding materials for making pressed

pellets, their suitability as grinding aids and their properties

Figure 5: Binding materials



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2.6) Effectively Mixing the Sample with Binder

Figure 6 MU-K-Mixer

The binder must be homogeneously mixed with the sample to ensure a mechanically stable

pressed pellet. Manually, this can be performed in a mortar, for example, by grinding the

sample and the binder with the pestle for a longer period of time.

More effective and timesaving is the use of an electric mixer such as the MU-K-Mixer. The

sample is homogenously mixed with the binder and a mixing ball in a disposable cup within 1-

2 minutes. It is absolutely necessary to remove the mixing ball from the mixture before

pressing!

This guarantees that the binding material is evenly distributed throughout the entire sample

and, thus, that during pressing, the pressed pellet is stable in all areas.



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2.7) Assembly of the Pressing Die

The pressing die for the PR-15t, PR-25N and PR-25A press is assembled as follows:

Figure 7: Assembly of the pressing die

When the pressing process is finished, remove the die from the press, turn it over and set it

down. Remove the bottom plate and carefully press down on the pressing die housing with

the balls of the hands. Caution: Danger of pinching!

It is also possible to place the pressing die upside down into the press, remove the bottom

plate and replace it with the WZ-0024.

Figure 8: WZ-0001

Then carefully lower the spindle by hand until the pressed pellet is pressed out of the die.

First, the press housing is placed onto the base plate.

Then the first pressing plate is placed into the housing, the sample

material introduced and then the second pressing plate is placed on

top. Make sure that the polished sides face the sample. To prevent

contamination, both pressing plates should be fit with pellet films (TF-

PF-32500 for 32 mm or TF-PF-40500 for 40 mm).

Finally, the pestle is placed into the die and the entire pressing die is

placed into the press. The press spindle is lowered until it touches the

pestle and can no longer be lowered by hand. The pressing process

can be started.



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2.8) Using Accessories: Pellet Films, Alu-Cups and Pressing Rings

Our pressing films (pellet films) prevent contamination and sticking of pellets onto the

pressing die.

The aluminum cups are ideal for unstable samples and for the labelling of samples.

The rings for pressed pellets are available in varying diameters and for different presses. They

are suitable for the stabilization of samples and are used for automatic and manual sample

preparation.

Figure 9: Pellet film Figure 10: Alu-Cups Figure 11: Pressing rings

2.9) Pressing Procedure

Figure 12: Pressing procedure Figure 13: Software documents pressing procedure

Pressure is built up during pressing. This occurs either manually using a lever or

automatically until the required pressure is reached.



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2.10) Removing the Sample

We recommend the WZ-0004a suction tool to easily and cleanly remove the sample from the

pressing die.

Figure 14: WZ-0004a suction tool

2.11) Analytical Results

The following table shows, as an example, the results for multiple preparations of a cement

sample with binder.

Table: Preparation of the cement sample CEM V02 as a pressed pellet with CEREOX® in a ratio

of 4:1.

Mass% Na Mg Al Si P S Cl K Ca

Prep. #1 0.206 2.60 3.70 12.38 0.040 1.35 0.045 0.79 39.24

Prep. #2 0.212 2.63 3.74 12.44 0.041 1.35 0.044 0.76 39.09

Prep. #3 0.221 2.66 3.78 12.47 0.042 1.36 0.044 0.75 38.92

Prep. #4 0.206 2.61 3.71 12.39 0.042 1.34 0.047 0.76 39.22

Average 0.211 2.63 3.73 12.42 0.041 1.35 0.045 0.76 39.12

Std. dev. 0.007 0.03 0.04 0.04 0.001 0.01 0.001 0.01 0.15

Mass% Ti V Cr Mn Fe Zn Sr Ba

Prep. #1 0.23 0.0091 0.0039 0.139 1.26 0.0123 0.069 0.040

Prep. #2 0.23 0.0093 0.0038 0.139 1.25 0.0128 0.067 0.045

Prep. #3 0.24 0.0098 0.0031 0.137 1.27 0.0133 0.067 0.045

Prep. #4 0.24 0.0099 0.0038 0.138 1.28 0.0132 0.069 0.045

Average 0.23 0.0095 0.0037 0.138 1.27 0.0129 0.068 0.044

Std. dev. 0.01 0.0004 0.0004 0.001 0.01 0.0005 0.001 0.002



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3.) Special Applications:

3.1) Pressed Pellets for Small Quantities (BOREOX Backing)

This is suitable for small quantities (approx. 0.5 g sample or less available) when a normal

pressed pellet is not possible.

The pressing die is assembled whereby the lower pressing plate is placed into it with pellet

film. Use the filling tool (PR-CD-BA32/ PR-CD-BA40) to introduce the sample. Distribute the

sample evenly with a careful twisting motion of the pestle. The aim is to achieve a flat and

uniform layer of material. Then fill in 5 to 7 g BOREOX, remove the filling tool, put in the upper

pressing plate and press.

A stable pressed pellet with a thin, but undiluted sample layer results and can be used for XRF

like a normal pressed pellet.

3.2) Using WC Pressing Plates

When analyzing very hard samples such as quartz, pressing can lead to contamination of the

sample by the elements Fe and Cr. In addition to employing pellet films, this can be

prevented by using tungsten carbide (WC) pressing plates. The table below shows the results

for Cr and Fe in sand pellets prepared once with hardened steel pressing plates and once

with WC pressing plates. The comparison reveals how strongly the pressed pellet is

contaminated with Fe and Cr.

Figure 15: Filling tool PR-CD BA32/40 Figure 16: Pressed pellet with a layer of sample

Figure 17: Breaking the sample shows the thin layer



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4.) Additional Information:

4.1) Storage and Use of Pressing Dies

The pressing die is made of hardened stainless steel to ensure its robustness. To prevent the

surface from corroding, it is important to remove all fingerprints, sample material or similar.

The pressing die must be stored in a dry environment. Any corrosion that does occur is only

on the sample surface and can be easily removed by polishing. It does not constitute a quality

defect.

The surface quality of the pressing plates has only a limited influence on the XRF results. As a

rule, the grain size of the powder is larger than impairments to the surface quality, i.e., as long

as scoring and pitting on the surface are smaller than the grain size of the powder, they have

no influence on the XRF measurement. Even in cases in which the pressed pellet displays

patterns or color progressions, we have not been able to establish any analytical influence.

Table: Comparison of the Cr and Fe

contamination when pressing sand

with pressing plates made of steel

versus tungsten carbide.



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4.2) Reusing the Rings for Cost Optimization

The rings can be reused for cost optimization and to improve sustainability. To do this, place the ring into the tool provided, place the pestle on the sample and then carefully knock out the sample with a hammer.

5.) Summary

When making pressed pellets, selection of the tools is a deciding factor for the quality

achieved. Mechanical stability is attained when a suitable binding material is used, whereby

efficient mixing of the sample and binder using a mixer is crucial. A whole range of manual

and automatic presses with pressing dies for various diameters is available for the pressing

procedure.

Ultimately, the good reproducibility in the example presented here demonstrates what

modern press technology can accomplish today.

Figure 18: Place the ring in the tool

Figure 19: Placement of the pestle

Figure 20: Knock out the sample with a hammer



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Figure 21: Summary of a pressing procedure

Literature

[1] Rainer Schramm, Röntgenfluoreszenzanalyse in der Praxis, korrigierte Auflage II,

FLUXANA (2017).

[2] www.fluxana.com



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